PROJECT SUMMARY/ABSTRACT In most neurodegenerative diseases including Alzheimer's disease (AD), specific proteins that are normally soluble, aggregate in either the intra- or extracellular space of the brain. In AD, the aggregation of amyloid-? (A?) appears to initiate disease pathogenesis and the aggregation of tau in specific brain regions is associated with neurodegeneration. Understanding factors that lead to protein aggregation and their spread through specific neural networks will likely provide important insights for development of new treatments. One factor that influences the likelihood that A? or tau will aggregate is concentration. Prior to and over the first 4 years of this PPG, project 1, working with the other PPG investigators, has produced strong evidence that something associated with the sleep wake cycle regulates interstitial fluid (ISF) A? levels at least in part via influencing synaptic activity. Further, manipulations that influence the sleep wake cycle that are linked with increasing or decreasing ISF A? acutely also increase or decrease A? deposition chronically if such changes occur over longer periods of time. While tau is a predominantly a cytosolic protein, we also found that it is present in the ISF and that its levels there can be regulated by excitatory synaptic activity. A key concept that has emerged in neurodegenerative diseases is that certain proteins that aggregate, such as A? and tau, appear to spread within the brain. Once aggregation occurs in one region, protein aggregates will often next appear in another brain region that is in a synaptically connected network. There is strong evidence in AD and in animal models that A? aggregation in some way drives the progression and spread of tauopathy within brain networks. This spread of protein aggregates may occur via a prion-like mechanism. In prior studies of the sleep/wake cycle, we performed manipulations that affect more than just sleep (e.g. stress) and did not specifically affect slow wave sleep. Some important questions remain. Does direct neural manipulation of wakefulness and slow wave sleep have the same effects we have previously seen in regard to A?? Is ISF tau, tau pathology, and tau spreading acutely and chronically affected by the sleep/wake cycle? How does A? influence ISF tau, tau pathology, and tau spreading in the context of changes in the sleep/wake cycle? We hypothesize that ISF A? and A? pathology is strongly affected by the sleep wake cycle and that the ability of A? to drive tauopathy occurs in part via effects of the sleep wake cycle influencing trans synaptic spread of tau aggregates. This hypothesis will be tested in these aims. Aim 1: To directly manipulate slow wave sleep and wakefulness via Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and determine the acute effects on ISF A? and tau. Aim 2: To determine the effects of modulating the sleep/wake cycle chronically via DREADDs and other methods on A? pathology, tau pathology synaptic integrity, network function, sleep, and behavior in APP/PS1?E9 mice +/- human tau. Aim 3: To determine the effects of modulating the sleep/wake cycle via DREADDs and other methods in a tau spreading model in the presence and absence of A?.